Internal combustion engines, including diesel engines, gasoline engines, and natural gas burning engines, create and emit a variety of different pollutants during operation that may be harmful to the environment and to human and animal health. These air pollutants can include, for example, oxides of nitrogen such as NO2 and NO3, commonly referred to as NOx. Due to increased environmental awareness, including government mandated emissions regulations and control, many manufacturers of internal combustion engines have taken measures to reduce the amount or effect of the pollutants that are created. Some of these measures are incorporated into the exhaust system associated with the internal combustion engine to remove, trap or chemically react with the pollutants being exhausted from the engine.
One type of exhaust treatment measure that reduces NOx through a chemical reaction is known as Selective Catalytic Reduction, commonly referred to as SCR. In the SCR process, a gaseous or liquid reductant agent is introduced to the exhaust system where the reductant agent can intermix with the exhaust gasses or it can be adsorbed onto a catalyst located in the exhaust system downstream of the internal combustion engine. A common reductant agent is urea, though other suitable substances such as ammonia may be readily used in an SCR process. The NOx pollutants can react with the reductant agent and the catalyst such that the NOx is converted into environmentally benign nitrogen (N2) and water (H2O).
Conversion of the reductant agent and the NOx is dependent upon a number of factors including the quantity of NOx produced, the exhaust temperature and the duration of the reaction before the exhaust gasses exit the SCR system. These factors also frequently change with the operating conditions of the internal combustion engine, resulting in either too little reductant agent being supplied and unacceptably low NOx conversion or too much reductant agent being supplied with the consequence of unconverted agent being expelled to the atmosphere, a phenomenon known as slippage. Another negative effect of introducing too much reductant agent is the formation of deposits of the agent within the exhaust system. Various systems and methods now exist to control the amount and/or rate of reductant agent injection in order to account for these variables and factors and maximize conversion efficiency.
One such system is described in U.S. Pat. No. 7,178,328 (the '328 patent), titled System for Controlling the Urea Supply to SCR Catalyst, which describes an electronic control module for controlling the SCR system. The control module measures a plurality of factors including exhaust temperature, intake airflow, and NOx quantities. From these factors, the control module calculates an injection strategy that may involve intermittent dosing of reductant agent to the exhaust system. In the '328 patent, intermittent dosing involves injecting excess reductant agent to the SCR catalyst followed by a period of delay during which the reductant agent converts with the NOx to maximize conversion efficiency.
The present disclosure is directed at overcoming one or more of the deficiencies described above and/or other problems present in the art.